This invention relates generally to frequency modulation (FM) radio systems, more particularly, systems for receiving all desired broadcast messages from multiple FM carriers of unequal amplitude. In addition, an inherent feature of the present invention is that all suppressed signals, both carrier and modulation, are recoverable to substantially the same extent.
In a conventional frequency modulation (FM) radio receiver, the demodulator circuit suppresses weaker FM signals. The receiver is said to be captured by the strongest signal and all presently known FM demodulators exhibit this "capture effect." Capture effect, a weak-signal-suppression characteristic, is a well-known property of frequency modulation theory
The prior art includes work by E. J. Baghdady as described in "Signal Cancellation Techniques for Capturing the Weaker of Two Co-Channel FM Signals", Electro-Magnetic Wave Propagation, 1960, pages 183-207, Academic Press. Two signal cancellation techniques were discussed, namely, (1) dynamic trapping and (2) feed-forward.
Dynamic trapping attempts to reduce the amplitude of the stronger signal by tuning an electrical bandpass filter. The filter reduces the signal strength of the stronger signal so that it becomes the weaker signal. A conventional demodulator is then used to recover the message from its dominant input, which was originally the weaker desired signal. The theoretical performance of the dynamic trapping technique is uncertain, since it appears to require that the concepts of instantaneous frequency and Fourier transform frequency be equated. Therefore, Baghdady relies on experimental results.
The feed-forward technique uses signal suppression provided by narrow band limiters. Such limiters are also difficult to analyze mathematically and again, Baghdady relies on experimental results for supporting this technique. By the very principles of their operation, neither dynamic trapping nor feed-forward function as intended when the instantaneous frequency of the applied FM signals are equal or approximately equal.
The prior art also includes U.S. Pat. Nos. 3,226,646--Ludwig, 3,753,123--Carpenter, et. al, and 4,739,518--Bickley, et. al. All of these references attempt to recover information from a weaker signal in the presence of one or more stronger signals by signal cancellation and related filtering techniques.
In Ludwig, a cancellation bridge is used for cancellation of interfering signals and employs a strong signal tracking filter for isolating the interference. The tracking filter incorporates a high signal capture-type demodulator, which provides an instantaneous analog signal for controlling coincidence of the instantaneous center frequency of an electronically tunable filter with the frequency of the undesired stronger signal. Only one output, namely the demodulated weaker signal, is obtained. Again, the concepts of instantaneous frequency and Fourier transform frequency are equated to explain the operation.
Carpenter, et. al., describes a system for subtracting unwanted signals from input signals to provide error signals at the output. This reference also teaches use of a phase-locked loop (PLL) in signal extraction means for producing an estimate signal, which includes any incidental amplitude variations of the carrier signal, and which is vectorially subtracted from the input signal. More than one estimate signal may be derived for vector subtraction and, since each extractor remains locked on its own signal, the effect of removing one or more input signals by such subtraction has little or no effect on the remaining signals. Cancellation only of undesired signals is provided
Finally, in Bickley et. al., the capture effect of a limiter is used to detect a desired signal received with an interfering signal at nearly the same frequency as the desired signal but at significantly greater amplitude. In the described system, a gain-controlled amplifier provides a constant amplitude signal having the amplitude of the interfering signal portion equal to the amplitude of the interfering signal portion produced by the limiter. The constant amplitude and limiter signals combine through a subtraction operation to effectively cancel the interfering signal while causing only small attenuation of the desired signal. Again, only suppression of undesired signals is described.
None of these references apparently appreciate other advantages and uses of capture effect of FM demodulators. In addition, while Carpenter, et. al, uses a phase-locked loop (PLL) in extracting signals from input signals, none of the references appreciate other advantages and uses of phase-locked loop technology for discriminating more than one signal component of the input signal.